1. Field of the Invention
The present invention is directed in general to the field of electric circuits. In one aspect, the present invention relates to a high voltage switch circuit.
2. Description of the Related Art
In electronic control systems which include drive load devices, transistor-based load drivers or switches are used to drive the load devices. Ideally, the transistor-based switch generates a predetermined high level output voltage with a minimal voltage drop, while minimizing the output current while generating a predetermined low level output voltage. However, such switches typically include additional current sensing circuits for purposes of measuring the actual load current, protecting the transistor driver, optimizing load control, and/or performing diagnostics. However, such additional current sensing circuits can impair the performance of the transistor switch (e.g., by introducing lossy elements), can add to the manufacturing cost and complexity, and can introduce errors in the switch performance.
To illustrate the design and operation of conventional current limited switches, reference is now made to FIG. 1(a) which depicts an exemplary transistor-based switch circuit 1 that includes a simple transistor switch 4 (M1) is connected in series between a first reference voltage (e.g., battery voltage Vbat) and an output (Vout), and that is gated across a resistive load 2 (R1). While the transistor-based switch circuit 1 has a very low voltage drop across the first transistor 4 (M1), it will be appreciated by those skilled in the art that there are significant variations in the on-resistance (Rds-on) and output current (Io) which depend on the Vbat voltage, the manufacturing process variations, and temperature variations. These variations over the operating range of the switch circuit 1 can be problematic in certain applications, as illustrated in FIG. 2 which depicts, at line 11, a simulated plot of the output current (Io) from the switch circuit 1 which increases as the output voltage (Vout) decreases and the battery (Vbat) shows, the output current (Io) from the switch circuit 1 is quite high when the output voltage (Vout) is low, leading to current consumption and uncontrolled current (unpredictable amount of current) that is undesirable with low power applications.
To compensate for these variations, additional circuitry is typically included to provide optimal drive capability while offering protection against voltage or current fluctuations. For example, FIG. 1(b) depicts a current mirror circuit 3 that includes a tracking or tracing branch (including a first transistor 4 (M1) that is connected between the first reference voltage (Vbat) and the output load (Vout)). The current mirror circuit 3 also includes a reference branch, including a second transistor 6 (M2) and resistive load 2 (R1) supplying a current IR1, that is connected between first and second reference voltages (e.g., battery voltage Vbat and ground). The gates of the first and second transistors M1 and M2 are connected to each other and to the resistive load 2 so that the output current (Io) which crosses the output load (Vout) tracks the current IR1 from the resistive load 2. While the current mirror circuit 3 generates a limited output current as the output voltage (Vout) is lowered, the voltage drop is higher than the transistor-based switch circuit 1, resulting in a lower output current and a higher equivalent output resistance for this operation region. This is illustrated in FIG. 2 which depicts, at line 13, a simulated plot of the output current (Io) from the current mirror circuit 3. As shown by comparing the output current plots 11, 13 in the area where the voltage drop is close to zero (e.g., Vo=9-10 V), the output current is smaller, meaning that the current mirror circuit 3 has a higher series on-resistance value (Rds-on).
This reduction in the output current can be partially corrected to form a high side switch with auto limited current by adding an activation transistor to the reference branch. This is illustrated with the modified current mirror circuit 5 shown in FIG. 1(c) where the reference branch includes a current mirror activation transistor 8 (M3) connected in series between the second transistor 6 (M2) and resistive load 2 (R1) and gated by the output voltage (Vout). The operation of the modified current mirror circuit 5 is controlled by the drain-to-source voltage drop (Vds) over the first transistor 4 (M1) to provide the controlled current performance of the simple current mirror circuit 3 in the smaller output voltage regions, and to also provide the lower series on-resistance value (Rds-on) performance of the transistor-based switch circuit 1 in the higher output voltage regions (low output voltage drop region). In particular, when the Vds of the first transistor 1 (M1) is higher than the threshold voltage of the current mirror activation transistor 8 (M3), the current mirror activation transistor 8 (M3) is active or conductive, resulting in current IR1 through the R1 resistor which turns the first and second transistors M1/M2 ON to work as a current mirror. However, when the Vds of the first transistor 4 (M1) is lower than M3 threshold voltage, the current mirror activation transistor 8 (M3) is not conductive (e.g., turned OFF) so that there is no current through the R1 resistor, and as a result, the gate-to-source voltage (Vgs) of the first transistor 4 (M1) is set to the first reference voltage (e.g., Vbat), in which case the modified current mirror circuit 5 works as a simple transistor switch. These two performance regions (current mirror and switch) for the modified current mirror circuit 5 are depicted in FIG. 2 which depicts, at line 15, a simulated plot of the output current (Io) from the modified current mirror circuit 5. As shown, the simulated plot line 15 includes a transition region 16. Below the transition region 16 (e.g., where the output voltage is less than approximately 5.2 V), the modified current mirror circuit 5 works as a current mirror, while above the transition region 16 (e.g., where the output voltage is greater than approximately 8.8 V), the modified current mirror circuit 5 works as a simple switch. However, in the transition region 16, there is a maximum or peak output current in the current plot 15 for the modified current mirror circuit 5 that is higher than the output current 13 for the simple current mirror current 3. This peak output current creates a performance problem in applications where the modified current mirror circuit 5 is used in an application that has a low power mode.
Accordingly, a need exists for an improved transistor-based switch with limited current, minimal voltage drop, and/or reduced output current peak to overcome the problems in the art, such as outlined above. Further limitations and disadvantages of conventional processes and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the drawings have been schematically illustrated, and where considered appropriate, reference numerals have been repeated among the drawings to represent corresponding or analogous elements.